International Residential Code - Collar Tie Conflict? 1

[MCurry]

Referring to IRC (2000 or 2003) Section R802.3.1;
The third sentence of the second paragraph states "Where ceiling joists or rafter ties are not provided at the top plate, the ridge formed by these rafters shall also be supported by a girder designed in accordance with accepted engineering practice." The first sentence of the paragraph limits this restriction to conditions where ceiling joists are not parallel to rafters.

The way I read this, any condition with ceiling joists located higher than the top plate (say a cathedral ceiling) will require a structural ridge beam, even if the ceiling joists/rafters meet the span requirements of IRC span tables R802.5.1 (1) through (8).

Have you all read this the same way?

 

[pcronin]

If you have a vaulted ceiling, typically it is a tray ceiling where the ceiling joists frame into the roof rafters above the sill plate. If this is the case, the ceiling frmaing is parallel to the roof framing and that section of the code does not apply.

However, you need to make sure you can develop the reaction of the ceiling framing into and through the roof rafter to the sill plate. You also need to check how much spread you will have at your sill elevation, which may cause cracking in your finishes and the roof to sag. This anaylsis is more complicated than pulling member sizes out of typical charts. If you are not comfortable with the anaylsis procedure, you should ask a structural engineer.

 

[MCurry]

thanks pcronin. I am a structural engineer, sorry for not saying so. I understand the structural issues, I'm wondering if others see this as a conflict with the IRC.

 

[Whodapookie]

I think the code guys intended for the ridge beam to be structural (trib area = 1/2 the rafter span) when no collar ties are present, as in cathederal ceilings. The reasoning is there are no ceiling joists to take away the horizontal force that collects at the walls which roof trusses have with both flat and sloped bottom chords (as in sissor trusses).

I don't see a conflict but my IRC is at the office. If I see one, I'll post it.

Below is a link to an interpritation by the state of Connecticut Building Inspector.

http://www.ct.gov/dps/lib/dps/offic...pretations/2006/i-02-06.pdf#search="R802.3.1"

 

[RARWOOD]

I found the link posted by Whodapookie very interesting. I was surprised that the code interpretation answer to question #1,didn't address the issues rasied by pcronin above.

Structurally a rafter ceiling joist system is an three hinged A-frame with a horizontal tie. When you raise the ceiling joist you start to develop a horizontal thrust at the top of the stud wall. In a lot of cases after you have raised the joist 2'-3' the thrust is high enough that the stud walls are unable to resist the horizontal thrust.

 

[Whodapookie]

Section 802.3.1 in the 2006 IRC was rewritten and is easier to understand. My interpretation below...

A structural ridge IS NOT required if the ceiling joists are parallel to the rafters, connected per table T802.5.1(9), and the adjustment factor from footnote "A" of Table R802.5.1(1) & (9) is used (Hc/Hr < 1/3) This was 2/3 in the 2003 IRC, but is now 1/3 in the 2006 IRC.

A structural ridge IS required if the ceiling joists are located more than 1/3 the height of the rafter height.

Also, a structural ridge is required if no ceiling joists or rafter ties are used.

I hope this helps. RARSWC is correct that this is an A frame system with a horizontal tie. I have seen in the field where the walls were pushed out because the builder did not design the ridge beam as structural for a cathedral ceiling. You definatly want to account for the horizontal thrust produced when the system deflects!

 

[Ron]

It's interesting that in that interpretation, if you took that premise farther, it implies that if a ridge beam is sufficiently stiff, no lateral thrust would exist.

I am currently evaluating the failure of one with no collar ties, and the ceiling joists do not tie to the rafters, just the walls. Lateral thrust appears to have overloaded the rafter-to-top plate connection. The ridge beam was a 1x8.

 

[RARWOOD]

Ron

I am not sure how to interpret your posting. Are you questioning wheter or not there is a lateral thrust if you have a large enough ridge beam?

I am confused because you refer to an 1 x 8 as a ridge beam. Athough as in your case, I have seen contractors and engineers try to use 1 x 8's as ridge beams. The 1 x 8 is intended to be a nailer and to provide a flat surface for the rafters to bear against.

 

[MCurry]

Whodapookie - I haven't seen the 2006 IRC yet, but it sounds like it clarifies this language a bit. Thanks.

Ron - with a properly sized ridge beam and no ceiling joists or collar ties, there should be no thrust. It sounds like you're saying the ceiling joists are not directly connected to the rafters, but to the top plate instead. I see two possible problems here - 1. if the rafters are not connected to the plate, they will try to slide outward. 2. If the rafters are connected to the plate, then the horizontal thrust is resisted by the top plates in bending between ceiling joists. Depending on the spacing of the ceiling joists and the wall thickness, the top plate could easily be overstressed.

 

[Ron]

Sorry for the confusion, but two separate subjects. The first is related to the interpretation. While a sufficiently stiff ridge beam limits lateral thrust, that can only occur if the ridge beam has fully engaged end supports and really only serves as a prop for the rafters. Otherwise the whole unit (all rafters) have lateral thrust.

In the second case (evaluation of a failure), the rafters were only attached to the top plate and there was significant lateral thrust that could not be resisted by the connection of the rafters to the top plate and by the lateral restraint offered by the joist attachment to the wall studs. In some places the rafter connection failed, while in others the wall pushed out. The ridge beam was a 1x8, but because of its size and the loading placed on it, had significant vertical deflection, thus not absorbing the load as would be desired to mitigate the thrust.

 

[Whodapookie]

Ron,

1st subject: Yes, the ridge must have posts or solid bearing that transfers 1/4 the area of the roof the ridge beam is supporting at each end. The rafters must be attached to the ridge beam to account for the vertical (I prefer sloping hangers) and horizontal force (usually a strap tie above the ridge beam). There are many ways to make this connection.

2nd subject: I do not consider a 1x8 a structural ridge. I could almost bet the rafters were toe-nailed to the 1x8 and nothing else. Essentially, the rafters were "leaning" against each other and creating a very large horizontal thrust on the wall. They are lucky it did not collapse and hurt someone.

I helped review a gym in the 90's made of timber trusses from the 1920's that had a vertical deflection of 6"-10" at the ridge among many other issues. Needless to say, the recommendation was replacement (which the school did). Moral of the story: Even trusses create this thrust and one side should be connected with a metal connector (Like Simposon Strong-Tie's TC24) that allows horizontal movement on one of the walls.

 

[Ron]

Whodapookie...it did collapse. I agree...the ridge beam was not an adequate structural member. The rafters were toe-nailed at the ridge and at the top plate...both inadequate.

This was a house built in the 1930's.

 

[connect2]

Interesting enough, fairly straight forward analysis as RARSWC states. Ron the roof lasted 76 years, assuming it was snow that caused it to collapse, it certainly lasted a 1/50 return period. Perhaps it collapsed for another reason, issues of current code requirements aside and ridge beams in bi-axial bending etc, a 1"x8" ridge beam, agreed not.

 

[Ron]

Connect2...it collapsed from construction materials loaded on the roof. Big question is whether it collapsed because structure was inherently deficient but had never received loads it should have been designed for, or if the load applied would have also collapsed a structure that was sufficiently designed and constructed.